In an internal combustion engine which employs intake and exhaust valves, the valve closing velocity is one of the most critical valve train performance parameters. Excessive valve closing velocity can cause such durability problems as high valve seat recession, valve stem stretching, and valve head wear. High valve closing velocities can also cause performance problems, such as valve "bounce" after closing, and high noise levels.
These problems have traditionally been treated in the art by grinding cam shaft lobes such that the valve decelerates as it approaches its seat, and then closes with relatively low velocity. This prior technique has proven quite satisfactory for conventional engines with fixed valve train geometry. Recently, however, hydraulic valve lifters have been employed with greater frequency in internal combustion engines so as to vary valve timing and duration of valve opening to thereby provide more optimum engine performance at various operating conditions (i.e. so-called "lost motion" systems). One such system employing hydraulic valve lifters is disclosed in U.S. Pat. No. 4,615,306 entitled "Engine Valve Timing Control System" of Russell J. Wakeman, issued Oct. 7, 1986 (the entire contents of this prior patent being expressly incorporated hereinto by reference, and referred to hereinbelow as "the Wakeman '306 patent").
In the Wakeman '306 patent, valve timing and valve opening duration are controlled via pressure pulses developed within the engine oil supply as a result of lifter operation. A pair of pistons defines therebetween a chamber in communication with a solenoid which controls the establishment of a hydraulic link between the pistons. As the lower piston of the pair is being moved up the cam's profile, oil is pushed out of the chamber into bleed passageways until such time as the lower piston's displacement is to be hydraulically transferred to the upper piston as dictated by an electronic control unit (ECU). At that time, the solenoid is energized thereby forming a solid hydraulic link coupling the motion of the lower piston to the upper piston which, in turn, actuates valve opening.
As one in this art will appreciate, such a "lost motion" system effectively eliminates the gentle closing ramp associated with the cam. Thus, high valve closing velocities and associated noise and valve durability problems (i.e. excessive valve wear), as has briefly been mentioned above, may result.
While in practice the low velocity opening ramp typically associated with a cam is relatively dispensable since the hydraulics of a "lost motion" system usually has adequate compliance, closing of the valve directly from the high-velocity closing ramp of the cam causes all of the predictable problems mentioned above since the gentle (i.e. low velocity) cam closing ramp effectively disappears when "lost motion" systems are employed. The conventional technique of providing a closing ramp having a more gentle slope (and thus lower valve closing velocity) is not an acceptable solution in "lost motion" systems since the gentle slope of the ramp would consume a considerable amount of cam rotation in order to achieve valve closure to the detriment of engine performance. What has been needed therefore is an engine valve whose motion is damped upon closure to its seat and thus is compatible with "lost motion" valve lifter systems. Moreover, it would be very beneficial if such an engine valve accomplished its damping functions passively (i.e. without requiring additional moving parts other than the valve per se) while yet providing for lower valve closing velocity.
There have been attempts in the past to hydraulically decelerate engine valves upon closure as evidenced by U.S. Pat. Nos. 3,209,737 entitled "Valve Operating Device for Internal Combustion Engine" of Isao Omotehara et al.; 3,938,483 entitled "Gasoline Engine Torque Regulator" of Joseph Carl Firey; 4,009,694 entitled "Gasoline Engine Torque Regulator With Partial Speed Correction" of Joseph Carl Firey; 2,827,884 entitled "Timed Actuator Mechanism" of Paul M. Stivender; and 3,865,088 entitled "Means for Hydraulically Controlling the Operation of Intake and Exhaust Valves of Internal Combustion Engines" of Heinz Links.
In Omotehara et al '737, a hydraulically actuated engine valve is disclosed as including a protrusion 15 formed on a plunger 12 so as to enter the buffer space 13 to force oil out from space 13 through a vent opening 14. This "venting" of oil continues until a needle rod 16 (coaxially formed with protrusion 15) is fully fitted within vent opening 14. Selective dimensioning of clearances between rod 16/vent opening 14 and protrusion 15/buffer space 13 is said to alleviate any impact occurring upon seating of the valve on its seat. There is no practical manner, however, of selecting that point in the valve's closing cycle when valve deceleration will occur since lengthening or shortening the "rod" 16 will have little, if any, impact upon the point in time when the vent opening is fully closed. Moreover, the coaxial disposition of the rod 16 and protrusion 15 renders it capable of use only with hydraulic actuators--as opposed to the use of more conventional mechanical valve actuators employing a valve spring.
Firey '483 and '694 each show an engine valve having a check valve which freely allows oil to flow therethrough during the opening stroke of the valve, but which prevents oil flow therethrough when the valve begins its closing stroke. Such a "dashpot" structure, however, retards the return of the valve to its seat continuously during the valve's entire downstroke to the possible detriment of engine performance.
The engine valve disclosed in Stivender '884 depends upon a complex rotary valve 118 which, in operation, closes the intake and exhaust portions of its cycle just before the plunger 78 has completed its valve opening and valve closing strokes, respectively. By adjusting valves 60 and 62, the rotary valve 118 (as experimentally accomplished on an external test stand) causes the plunger 78 to be decelerated at each end of its stoke before it strikes either of the dash-potting cushions 79, 79' and 81, 81'. While the rotary valve 118 does determine those points in the engine valve's opening and closing strokes where deceleration occurs, it does so at the expense of complex valving mechanisms clearly not suitable for use in today's sophisticated internal combustion engines.
Finally, the engine valve of Links '088 is capable of being hydraulically braked towards the end of either of its strokes by means of a collar 34 formed on work piston 9 which enters in one of the cavities 35 or 35'. The cavities 35, 35' are of a slightly larger diameter as compared to collar 34 so that oil escapes through the narrow clearance formed therebetween and thus cause a dashpot effect to be achieved.
As those in this art will appreciate, none of the prior proposals mentioned above would be suitable for use with a "lost motion" valve control system since there appears to be no precise manner in which one can predetermine that point in the valve's downstroke where deceleration will occur--without resort being made to complex valving structures employing unwanted additional moving parts and their concomitant reliability problems. Thus a simple passive valve deceleration system has been needed but such a need has not, to the best of the Applicants' knowledge, been satisfied.
In accordance with the present invention, however, an especially designed engine valve is provided having a modified valve stem that serves as a hydraulic valve spool--that is, the valve stem includes a section of larger cross-sectional diameter (as compared to the valve stem) so that a hydraulic damping chamber is formed in the valve guideway. The chamber is closed at one end by the sliding spool and at its other end by the valve guide of the smaller diameter stem. As the valve opens, the damping chamber increases in volume and, conversely, decreases in volume as the valve is closed. An oil feed passage and an oil bleed passage open into the damping chamber but are normally closed by the spool's peripheral surface (i.e. when the engine valve is seated). The feed passage is connected to the pressurized engine oil system and is opened at a predetermined point during the valve's opening cycle thereby allowing the damping chamber (with which the feed passage then communicates) to be filled with oil under pressure. The bleed passage, on the other hand, is arranged so that oil is discharged therethrough from the damping chamber to an area of low pressure (typically, within the valve cover) where the discharged oil is recovered by the normal engine oil recirculation system.
At a predetermined location in the engine valve closing cycle, the valve spool will cover, and thus close, the bleed passage thereby sealing it. As the engine valve closes further under the bias influence of the valve spring, the valve spool continues to displace oil (which early in the closing cycle passed through the bleed passageway but which now must pass through high restriction clearance spaces defined between the valve spool and the valve guideway, for example). Thus, upon closure of the bleed passageway, the velocity of the valve is significantly reduced due to oil "trapped" within the damper chamber--this trapped oil then escaping via the high restriction clearance spaces thereby to maintain this substantially reduced closing velocity of the engine valve during its closure to seat.
The location of that end of the bleed passage in communication with the damper chamber relative to the immovable wall of the damper chamber, moreover, establishes the precise point during the valve's closing cycle when motion-damping functions attributable to the structure of this invention will be initiated. This, in turn, permits the valve to be decelerated at any predetermined point of its downstroke which has the advantage of permitting an engine designer to incorporate the valve structures of this invention in a variety of valve opening systems (including "lost motion" systems) without deleteriously affecting engine performance as a result.
The valve is sealed against oil leakage by means of at least one pair of axially spaced-apart seal members. A drain port is defined in the cylinder head and includes an inlet end which communicates with the valve's guideway between the spaced-apart seal members at all times during the valve's stroke between its opened and seated positions. An outlet end of the drain port, communicates with a region of low pressure within the engine oil circulation system (typically, within the valve cover). In such a manner, any oil which may leak past the uppermost one of the seal members is directed, via the drain port, to a low pressure region of the engine oil circulation system. The lowermost one of the seal members, on the other hand, prevents oil leakage into the combustion chamber where it would deleteriously affect engine performance.
These as well as other objects and advantages of the present invention will become more clear to those skilled in this art after careful consideration is given to the detailed description thereof which follows.